With the throttle valves closed and engine at idle, fuel is metered by the idle tube metering orifice through the idle tube. Air from the idle air bleed is mixed with the fuel at the top of the idle tubes. This mixture of air and fuel runs down the channel and then mixes with air from the secondary idle air bleed and then discharged through the idle discharge holes. The quantity of fuel is metered by the idle mixture screw (idle needle valve). As the throttle valve is opened more, additional fuel is discharged from the upper idle discharge holes.
Rough Idle?
- Adjust idle mixture screw – use a vacuum gauge and adjust for smoothest idle (highest vacuum). Adjust at idle speed. Once the idle is over idle speed then the idle circuit is no longer in play.
- Idle vent dirty, out of adjustment (if so equipped).
- Idle needle worn or damaged. Look for any scores. Polish with a wire wheel.
- Throttle valves not complete closing, bent, sticking open, installed in the wrong bore, or upside down.
- Idle passages dirty, or plugged.
- Power piston vacuum passage plugged or dirty.
- Pump discharge check – out of round, stuck, distorted spring.